This invention relates generally to semiconductor device fabrication, and more particularly, to processes for fabricating a process structure, such as a reticle, mask, mask blank, etc., with reduced defects.
The electronics industry continues to rely on advances in semiconductor technology to realize ever higher-functioning devices in more compact areas. For many applications, realizing higher-functioning devices requires integrating a larger and larger number of electronic devices onto a single wafer. As the number of electronic devices per area of wafer increases, the manufacturing processes become more intricate.
One of the process steps encountered in the fabrication of integrated circuits and other semiconductor devices is photolithography. Generally stated, photolithography includes selectively exposing a specially-prepared wafer surface to a source of radiation using a patterned template to create an etched surface layer. Typically, the patterned template is a reticle, which is a flat, glass plate that contains the patterns to be reproduced on the wafer.
The industry trend towards the production of integrated circuits that are smaller and/or of higher logic density necessitates ever smaller line widths. The resolution with which a pattern can be reproduced on the wafer surface depends, in part, on the wavelength of ultraviolet light used to project the pattern onto the surface of the photoresist-coated wafer. State-of-art photolithography tools use deep, ultraviolet light, with wavelengths of 193 nm, which allow minimum feature sizes on the order of 20 nm. Tools currently being developed use 13.5 nm extreme ultra-violet (EUV) light to permit resolution of features at sizes below 30 nm.
Extreme ultraviolet lithography (EUVL) is a significant departure from the deep, ultraviolet lithography currently in use today. All matter absorbs EUV radiation, and hence, EUV lithography takes place in a vacuum. The optical elements, including the photo-mask, make use of multi-layers, which act to reflect light by means of interlayer interference. With EUV, reflection from the patterned surface is used as opposed to transmission through the reticle characteristic of deep, ultraviolet light photolithography. The reflective photo-mask (reticle) employed in EUV photolithography is susceptible to surface irregularities, contamination and damage to a greater degree than reticles used in conventional photolithography. This imposes heightened requirements on reticle manufacturing destined for EUV photolithography use. For example, any surface irregularity in the reticle could compromise the reticle to a degree sufficient to seriously affect the end product obtained from the use of such a reticle during processing.